In Act 1, Scene 3, Ross and Angus, two noblemen, arrive to tell Macbeth that he has been named Thane of Cawdor by King Duncan:
And, for an earnest of a greater honour,
He (Duncan) bade me, from him, call thee thane of Cawdor:
In which addition, hail, most worthy thane!
For it is thine.
Puzzled by this news, Macbeth requests to know whether this news is valid or not, because he assumes that the current Thane of Cawdor is still alive, so he questions what has been said. His suspicion is dispelled by Angus, who proclaims that the former thane of Cawdor conspired with the King of Norway so that they could defeat Duncan. As a consequence, he is imprisoned and soon to be executed:
Who was the thane lives yet;
But under heavy judgment bears that life
Which he deserves to lose. Whether he was combined
With those of Norway, or did line the rebel...I know not...
The news, announced by Ross and Angus, is of extreme importance because now, Macbeth starts believing in the prophecies of the witches. They called him Thane of Glamis (his current title), Thane of Cawdor (what he has just become), and the King. The new information he gains has solidified his secret desire to become king. This is the turning point in the play, because from now on, Macbeth will do anything he can to become king, blinded by the witches' prophecies.